The "atom:feed" element is the document (i.e., top-level) element of
an Atom Feed Document, acting as a container for metadata and data
associated with the feed. Its element children consist of metadata
elements followed by zero or more atom:entry child elements.

This specification assigns no significance to the order of atom:entry
elements within the feed.

The following child elements are defined by this specification (note
that the presence of some of these elements is required):

o atom:feed elements MUST contain one or more atom:author elements,
unless all of the atom:feed element's child atom:entry elements
contain at least one atom:author element.
o atom:feed elements MAY contain any number of atom:category
elements.
o atom:feed elements MAY contain any number of atom:contributor
elements.
o atom:feed elements MUST NOT contain more than one atom:generator
element.
o atom:feed elements MUST NOT contain more than one atom:icon
element.
o atom:feed elements MUST NOT contain more than one atom:logo
element.
o atom:feed elements MUST contain exactly one atom:id element.
o atom:feed elements SHOULD contain one atom:link element with a rel
attribute value of "self". This is the preferred URI for
retrieving Atom Feed Documents representing this Atom feed.
o atom:feed elements MUST NOT contain more than one atom:link
element with a rel attribute value of "alternate" that has the
same combination of type and hreflang attribute values.
o atom:feed elements MAY contain additional atom:link elements
beyond those described above.
o atom:feed elements MUST NOT contain more than one atom:rights
element.
o atom:feed elements MUST NOT contain more than one atom:subtitle
element.
o atom:feed elements MUST contain exactly one atom:title element.
o atom:feed elements MUST contain exactly one atom:updated element.

If multiple atom:entry elements with the same atom:id value appear in
an Atom Feed Document, they represent the same entry. Their
atom:updated timestamps SHOULD be different. If an Atom Feed
Document contains multiple entries with the same atom:id, Atom
Processors MAY choose to display all of them or some subset of them.
One typical behavior would be to display only the entry with the
latest atom:updated timestamp.